Ge-Si ratio for assessing the chemical weathering intensity at the paleocene-eocene boundary in ancient transuralian Basin

Abstract

© 2020 Tomsk Polytechnic University, Publishing House. All rights reserved. Relevance. Diatomites and their transitional differences formed in the Paleocene-Eocene marine basin are one of the most widespread in the near-surface occurrence of sedimentary rocks in the vast territory of the Trans-Urals. At the same time, with regard to the genetic aspects of such rocks, the discussion on the fundamental sources of silica for formation remains still relevant. An urgent task is to analyze the role of each of two factors in the origin of rocks - continental weathering and upwelling activities. The ratio of Ge to Si can potentially be considered as a tool for deciphering the source of silica in biogenic siliceous rocks, since germanium is delivered to the rock by incorporation into biogenic opal; Ge introduction into the rock from other sources during the diagenesis is not significant, which makes the results indicatively valuable. The main aim is estimation of the chemical weathering intensity at the boundary of the Paleocene and Eocene in the Trans-Urals based on the study of the ratio of Ge and Si in diatomites of different ages. The methods: field studies, X-ray fluorescence analysis, inductively coupled plasma mass spectrometry, scanning electron microscopy. The results. The paper contains the first results of estimation of chemical weathering intensity at the boundary of the Paleocene and Eocene in the Trans-Urals by analyzing the ratios of germanium and silicon in bulk diatomite samples and bioclastic preparations, where clay material was removed. The Ge/Si ratios in bioclastic preparations from the Paleocene (Brusyana section) and Eocene (Irbit deposit) differ almost twofold. The obtained values should be considered as mark that the intensity of chemical weathering undergoes serious changes at this boundary: intensive chemical weathering, strengthening of the hydrological cycle during the PETM period determined an increase in the supply of dissolved silica to the marine basin during the accumulation of biosiliceous rocks in the territory of modern Trans-Urals. The weakening of tectonic movements and land peneplenisation within the East Ural contributed to the weathering intensification. This provided an additional transfer of siliceous matter in the form of true and colloidal solutions to the sedimentation basin for the biosiliceous rocks accumulation. The amount of silica influx from land over a relatively short period of time became more noticeable in the overall balance of silica supplied to the sedimentation basin. For the same reason, i. e. the presence of an additional source of silicic acid, the gross SiO2 content in the Paleocene diatomites is slightly higher than in the Eocene.